Current transformer



July 26,1932. A. o. AUSTIN CURRENT TRANSFORMER Filed Oct. 17, 1929 3Sheets-Sheet l A TTORNEY July 26, 1932. A. o. AUSTIN f 1,868,481

' CURRENT TRANSFORMER Filed Oct. 17, 1929 s Sheets-Sheet 2 11v VENTOR A'TTOR NE y July 26, 1932. A. O. AUSTIN 5 CURRENT TRANSFORMER v v vFiled-Oct. 17,1929 3 Sheets-Sheet "3 Patented July 26, 1932 STATESPATENT crater.

. ARTHUR O. AUSTIN, 0. NEAR BARBERTON, OIIIO, AS SIGNOR, BY MESNEASSIGNMENTS,

TO THE OHIO BRASS COMPANY, OF IEANSFIELD, OHIO, A CORPORATION OF NEWJERSEY CURRENT TRANSFORMER Application filed October 17, 1929; SerialNo. 400,267.

This invention relates to an improved type of current transformers,particularly applicable to high voltage lines, and has for one of itsobjects the provision of a current transformer adapted for a large rangeof current values.

Another object is to provide a current transformer which willautomatically adjust itself to variations in the amount of currentflowing.

A further object is to provide a current transformer in. which thenumber of turns in the primary and secondary circuits may besimultaneously adjusted.

A further object is to provide a device of the class named which shallbe of improved construction and operation.

Other objects and advantages will appear from the following description.

The invention is exemplified in the combination and arrangement of partsshown in the accompanying drawings and described in the followingspecification, and it is more particularly pointed out in the appended 4claims.

In the drawings:

Fig. 1 is a fragmentary, vertical, sectional view showing a portion of acurrent transformer having one embodiment of the present inventionapplied thereto.

Fig. 2 is a fragmentary, vertical, sectional view with parts inelevation showing the top portion and reservoir of the bushing illus- V,trated in Fig. 1.

tions for the primary windings.

Fig. 9 is similar to Fig. 8 but with the switches in a differentposition.

In high voltage transmission lines or other conductors operating at highvoltages, it is frequently necessary and advisable to have an accurateindication of the current flowing in the conductor. One common methodused for obtaining an indication of the current, for the operation ofrelays, meters, tripping circuits or for other purposes, has been to usea current transformer. Where the voltages are low, efficient currenttransformers are readily made, giving practically any ratio and phaseangle with a high degree of accuracy. For high voltages, however, theordinary forms of current transformers are not well adapted, due to thefact that the large amount of insulation required between the conductorand the secondary and core or low voltage winding of the currenttransformer, requires considerable separation space. This separationnecessitates a long magnetic path with a high reluctance, increasing thecost of the transformer and giving one ofpoor characteristics.

One of the common points of applying current transformers of this typeis at bushings where they are placed in circuit breakers, ortransformers, or where the leads enter buildings as the bushing providesthe necessary insulation between the power conductor :and

ground, or the iron and secondary of the current transformer.

Where the usual bushing type of transformer is used, a single turn onlyis available unless the conductor is threaded through the bushing anumber of times, which at best requires two bushings, and inmany casesis not at all applicable.

In high voltage lines, the amount of current is frequently relativelysmall, hence, where the main conductor passes through a bushing type ofcurrent transformer but once, the ampere turns provided by this primarymay be too low to provide a transformer of desirable characteristics, orat best requires a transformer having a large magnetic circuit.

In my prior Patent No. 1,723,000, an improved type of currenttransformer is shown. In this type, the primary winding magnetizes acentral core so that even though the current is small in the powerconductor, a sufiicient number of ampere turns may be obtained forenergizing the core of the current trans.- former.

Where the current is small, the

q of) number of turns may be increased so that the same generalcharacteristics for the current transformer may be obtained for a highvoltage conductor for any given condition even though the current may bequite diiferent in the conductor.

In many instances, the current in the power conductor may vary greatlyfrom time to time. This is particularly true on interconnected circuitswhere the flow of energy is sometimes reversed. Where the amount ofcurrent in the primary covers a considerable range, it is highlydesirable that the curren transformer have good characteristics over aswide a range as possible. While the type of current transformer shown inmy prior patent, No. 1,723,000, makes it possible to provide a greatlyimproved current transformer over those heretofore applied to bushingsfor high voltage conductors, the characteristics may be further improvedif the volt ampere turns are approximately the same for the magneticcircuit over a large range of current. It is possible to obtain onimproved characteristic for the current transformer by changing thenumber of turns on the primary Winding when the current exceeds certainlimits. It is generally advisable to change also the number of turns onthe secondary side of the transformer so as to maintain a constantratio. here this is done, the range in magnetic flux in the core may beheld within much narrower limits even though the current varies over avery large range.

By providing the bushing primary with suitable windings, it is possibleto change the number of turns by a suitable switch. here it is desiredto maintain a constant ratio, the turns in the secondary must be changedcorres ondingly, which can be readily taken care of y any suitableswitch. In order to take advantage of the improved characteristicsgained thereby, it will be necessary to keep a careful watch of themagnitude of the current. The desired result, however, may be obtainedby the use of a suitable automatic switch or relay. In this automaticswitch or relay, the current in the primary may be caused to operate aswitch which will change the number of turns in the primary. Byconnecting the switch for the primary, with a suitable switch for thesecondary, the number of turns in the secondary may be changed tocorrespond simultaneously with the change of the number of turns in theprimary, thereby maintaining a constant ratio. Any suitable means forchanging both the primary and secondary simultaneously may be used. Thismeans may be entirely outside of the bushing or within the bushing asshown; the

important consideration being to change t e number of turns in theprimary and secondary simultaneously. It, of course is evident that thenumber of turns may be changed in the primary and the impedance in thesecond- 1 ,ses:4s1

ary circuit changed so as to obtain the desired result without changingthe number of turns in the secondary circuit.

As generally used, a current transformer will have a higher ratio onlighter currents and a lower ratio on heavy currents. This tends to iveinaccurate results and is objectionable for metering. The phase angle isalso much poorer where the magnetizing force of the primary is low. Inorder to ave'i d this difliculty, the primary may be provided withmultiple windings or taps. here the range in current is considerable,however, the characteristics of the transformer will not be good overthe entire range. This is corrected by providing a relay, or othermeans, which will automatically change the number of turns for both theprimary and secondary windings, so that a good working range will beprovided for the current transformer over a wide range of current in theprimary.

In the drawings, the numeral 10 designates the outer porcelain member ofa bushing having the conductor 11 extending therethrough. Battles 12 and13 surround the conductor 11 and a metallic screen or flux Controlmember 14:, preferably of magnetic material may be placed outside of thebaflie 13 and supported by a ring 15 clamped between the flanges 16 and17 secured to the upper and lower portions respectively of the outerhousing 10. Surrounding the conductor 11 ans. separated therefrom byinsulation 18 is a magnetic core 19 and a second layer of insulation 20is placed outside the core 19. Three insulated coils 21, 22 and 23 arewound, one inside the other, about the core 19; the coils beingseparated from one another by interposed insulation 24. The three coilsare substantially identical and each consists of a flat ribbon woundhelically about the core and having the ends thereof insulated from eachother and carried upwardly to the top of the bushing.

The lower end of the coil 23 is bent about the lower edge of heinsulation 24, as indicated at 25, and extends upwardly as shown bybroken lines at 26. The upper end of the coil 28 is bent upwardly at 27,and the two parts 26 and 27 are insulated from each other and extendupwardly side by side to the top of the bushing. The ends of the coils21 and 22 are carried upwardly in a like manner but the points aredisplaced angularly about the core from the ends of the coil 23 and fromeach other. The coils 21, 22 and 23 form the primary windings for thecurrent transformer and switching mechanism is provided for placingthese coils in the main circuit of the conductor 11, either singly orwith two or three in series. The secondary winding of the currenttransformer is shown at 28 and consists of three separate coils havingthe ends thereof separateincludes an automatically operated switch lever30, and a hand operated switching spider 31. The arm is held in theposition shown in Fig. 2 by a spring 32, and is secured to a shaft 33,journalled in a pedestal 34, and provided with a magnetic armature 35.The armature is controlled by a magnet 36 having a winding 37 in serieswith the main conductor. l/Vhen current of a prede termined amountpasses through the winding 37 the armature 35 will rotate the shaft 33to shift the position of the switch arm 30 in a clockwise direction, asviewed from Fig. 2.

The switch arm 30 carries the three contact blocks 38,39 and 40,eachblock being insulated from the switch arm. Each contact block isarranged to enter between contact blades carried on brackets 41, 42 and43 respectively, the blades being insulated from each other as indicatedat 44 in Fig. 6 and connected by their respective contact blocks whenthe block is in position between the blades. The spider 31 is mounted ona shaft 45, controlled by a lever 46. The spider consists of three armscarrying contact blocks 47, 48 and 49.

' The contact blocks 47 and 48 are electrically connected with eachother but the contact block 49 is insulated from the other two blocks.The block 49 is arranged to enter between a pair of blades 50 and 51,while the blocks 47 and 48 are arranged to engage single blades 52 and53 respectively. A pair of connecting rods 54 and 55 extend downwardlyfrom the lever 30 inside of the bushing 10, and are connected at theirlower ends to a cross arm secured to a shaft 56, journalled in astuffing box 57 extending through the wall of the housing 10. The shaft56, at its outer end, carries a mercury switch having three chambers 58,59 and 60. Each of these chambers, as shown in Fig. 7, carries a pair ofcontacts which may be connected or separated by tilting of the mercurychamber. Contacts are connected with the secondary windings L. of thecurrent transformer in a manner to be more fully described. A secondhand switch, shown diagrammatically at 61 in Fig. 7, is also connectedin the secondary of the transformer winding. The switch 61 in itsconstruction may be similar to the switch shown in Fig. 3. The operationof the transformer will be best understood by reference to Fig. 7 Thecurrent enters from the transmission line 62 and passes through anyusual form of terminal connection 98 to the relay winding 37, within thereservoir at the top of the bushing insulator.

When the switch blades 30 and 31 are in the positions shown in Figs. 2,3 and 7 the current flows from the winding 37 through the blade 50,contact 49, blade 51 and conductor 26 to the coil 23 upon the core 19and returns through the conductor 27 to the contact 52, and thencethrough the switch arm 31 to the conductor 63 to the primary winding 22,and back through the conductor 64 to the contact 65. The current thenpasses through the block 39 to the contact 66 and through the conductor67 to the primary winding 21 and back through the conductor 68 to thecontact 69. The current then travels through the block 38 to the contact70 and through lead 71 to the top of the conductor 11, which passes downthrough the bushing. seen that with the switches in this position, allthree of the primary windings, 21, 22 and 23, are in series with themain current of the bushing, and that the winding 37 of the relay isalso in series with the main conductor. now the current increases to apredetermined amount, the relay 37 will act against the spring 32 androtate the arm 30 so as to disengage the contact blocks 38' and 39 fromIt will thus be their switch blades, and to bring the contact ductor 26,winding 23, and conductor 27 to the contact 52 and thus through theconduc tor 63, coil 22, and conductor 64 to the contact 65. The contact65, however, is no longer connected to 66 so that the current passesthrough the jumper 74 to the contact 73,

block 40, and contact 72, and thus through conductor 71 to the mainconductor 11. In this position of the switch, it will be seen that onlytwo primary coils, 22 and 23, are

now in the circuit, and that the circuit leading 1 from the coil 21 isopen.

It should be noted that the contacts of the switch 30 are so arrangedthat the block 40 enters between the contacts 72 and 7 3 before theblocks 38 and 39 leave their respective con- I tacts, so that at no timeis the main circuit broken. If the current should again become weaker sothat the magnet 37 will no longer hold the arm 30 against the tension ofthe spring 32, the arm will be returned to itsoriginal position and thecoil 21 again in cluded in the circuit. It will be noted, however, thata somewhat smaller current will hold the arm in its shifted positionagainst the spring 32 than is required initially to shift the arm, sothat there will be no point at which the switch will be in unstableequilibrium between the two positions, and consequently no chattering ofthe switch and vibration between its two positions. The coil- 23may becut out of the circuit at the will of the operator by shifting the handswitch 31. When this switch is rotated in a counterclockwise direction,as shown in Fig. 7, the block 49 will be withdrawn from the contact 50and the block 47 withdrawn from the contact 53, so that both ends of thecoil23 will be left open. At the same time, the contact 48 will engagethe contact 53, so that current will flow from the relay 37, directly tothe conductor 63 instead of passing first through the coil 23. An arcinggap 99 is provided between the lead 71, connected to the conductor 11',and the incoming conductor 62. It will be seen that this gap will be inparallel with the primary turns of the transformer for all combinationsof these windings and will prevent subjecting these turns to excessvoltage due to abnormal surges on the line or other causes.

When the primary windings are connected, as shown in the drawings, thesecondary windings 28 will also all three be in series, as shown in Fig.7. The secondary windings are numbered 7 5, 76 and 77 respectively.Starting with the lower end of winding 75, as illustrated in Fig. 7,current passes through conductor 78 to contact 79, which engages block80 on the hand switch 81. The current passes through the block 80 tocontact 81 and conductor 82 to the primary winding 83 of a transformer,by means of which current is utilized. From the transformer winding 83,the current passes back through the conductor 84 to contact 85 on themagnetic switch unit 59 and thence to contact 86 and through conductor87 to the winding 77, and from winding 77 through conductor 88 tocontact 89, in switch unit 58. The current then passes to contact 90 andthrough conductor 91 to winding 76, and from winding 76 to conductor 92and block 93 to contact 94, and thence back through conductor 95 to thewinding 75. It will thus be seen that all three of the secondarywindings are in series with the trans former winding 83. Now when therelay 37 shifts the switch 30,.it will, at the same time, rock themercury contact units and raise both sets of contacts 89 and 90, and 85and 86 out of the pool of mercury, and at the same time immerse thecontacts 96 and 97. Current will now pass from the winding 75 throughthe hand switch and transformer winding 83 as before explained butinstead of entering the contact 85, it will enter the contact 96 because85 is now open. From contact 96, it will go through 97 to 91 and thencethrough winding 76 and back through the hand switch and contact 94 andconductor 95 to the winding 75, so that now only two windings, 75 and76, are in the circuit. Both leads from winding 77 are open. If the handswitch 81 is shifted, the coil 75 will be cut out of the circuit, aswill be readily apparent, leaving only the coil 76.

It will nowbe apparent that two'coils, are

provided in both the primary and secondary will be apparent that thenumber of turns adjusted automatically may be varied by increasing thenumber of switch contacts controlled by relay magnets.

In some types of bushings, it is desirable to provide a capacitance tapas well as a current transformer. The design of bushing shown in thedrawings is particularly adapted for obtaining the desired results. Inthis case, the proper ratio may be obtained by suitable taps in thesecondary of the insulating transformer without changingthe number ofturns in the secondary winding. he insulating transformer, the primaryof which is shown at 83, Fig. 7, in addition to having leads 100 and 101on the secondary, may have suitable taps 102 and 103 or others asrequired. lVhere these taps are used, it is not necessary to change thenumber of turns in the secondary. Switches like those illustrated forthe secondary circuit may be used for changing the taps on theinstrument or relay side of the insulating current transformer havingthe primary 83. Where the insulating transformer is used, the secondarywinding of the current transformer is insulated from ground by bafflesor any suitable means.

'The secondary winding of the current transformer forms a condenser withthe conductor or primary winding, thus providing an elect ostaticcoupling. using the insulating former with primary 83, the chargingcurrent flowing to the condenser provided between the secondary windingof the current transformer and primary winding or main conductor 11 canthen be made to flow through the primary 104 or" a step-down transformerprovide the necessary volt ampere capac y in the secondary 105 for theoperation of s nchroscopes, relays, charging batteries or ratio andphase angle correction for current transformers. 37 here the maximumvolt ampere output for the capacitance coupling is desired, the chargingcurrent be tween the secondary winding and the high voltage system in ybe further increased by electrically connecting metalized areas with onecoil of the winding 28 which will further increase the capacity couplingbetween the primary and secondary. The met alized area 110 should have abreak therein so as not to form a short circui turn for the transformer.When the increased metalized surfaces, electrically connected. to thesecondary winding 28, are used as controls or 2 same or different, ifdesired.

end shields adjacent to the secondary winding, the electrostatic control14 may be omitted. If however, it is desired to retain a magnetic return14, this may also be connected electrically to the secondary winding 28and 'metalized surface 110. In this case, the insulating battle 106 maybe considerably smaller than shown. It is necessary, however, to provideinsulation between the member 14 and ground by a suitable sleeve 107 orother means. It is evident that a bushing equipped with suitable currenttransformer may be used as a current transformer or for a currenttransformer and capacitance coupling, and 15 for no other purpose. Inthis case, the outer or insulating sleeve may be changed to meet theconditions.

The number of turns in the several primary windings 21, 22 and 23, maybe the In general, the larger the current the fewer the number ofturnsrequired. It is therefore frequently possible to provide onewinding with a few turns of large current capacity and other 25 windingshaving more turns to be used where the current is reduced. For instance,winding 22 may have twice the current carrying capacity of winding 21,and three times the current carrying capacity of winding 23. Anothermethod of providing a large current carrying capacity for the currenttransformer winding is to arrange the coils so that they may be used inseries or parallel arrangements. A simple form of this is shown in Fig.8 in which a primary composed of two coils, 108 and 109, are connectedin series. In Fig. 9 the two coils are shown connectedin parallelthereby doubling the current carrying capacity. The change from seriesto parallel arrangement may be effected automatically by the relay 37and switching mechanism as shown. Other combinations of windings may bereadily produced by suitable switching arrangement.

It is apparent that the useful electromagnetic flux is that which passesthrough both the primary and secondary coils in the same direction. Byproperly proportioning the core about which the primary is wound, alarge percentage of the flux may be made to thread both coils of thecurrent transformer. It is, however, not necessary that all the fluxthread both coils but rather that the ratio of flux threading both coilsto the total flux remain constant.

Where there is a long air path any change in the reluctance of the airpath, external to the bushing, may change the ratio of flux threadingthe primary and secondarycoils. WVhile in general this change in flux,is of little importance, there are some cases where such changes beundesirable. The re turn magnetic path provided by the shield 14 tendsto insure the direction of the flux 55 and to maintain the ratioconstant, as this magnetic shield will tend to screen the effect ofadjacent metal portions such as the bushing flange, transformer, orcircuit breaker cover. The ratio, however, may be made more nearlyindependent of outside influences by using conducting shields or turns..These shields or turns may be said to neutralize any stray field orflux which tends to take a path outside of their circumference. The

member 111 may be given a plating of copper or other low resistancemetal which will act as a short circuit turn. This layer will, ofcourse, be outside of the magnetic path provided by the member 14 andcore 19 and, in addition, a shield 113 made of copper or other goodconducting material may be made to act as a short circuit turneliminating the flux to a very large extent. It is evident that anyelectromagnetic flux which returns inside of this short circuit member113 will have no effect upon the current in this member. Even though themember 113 is not short circuited, it will tend to prevent the flow offlux through the same. It of course is not necessary that this member bein the form of a cylinder, but it may be of any form such as a series ofrings or turns of conductor or, if desired, the screening cylinder mayhave windows therein. Another method of screening the effect of outsidemetal is to coat with copper or other good conducting metal the innersurface of the bushing wall with a layer 112 so as to form a. shortcircuit turn. 7 This metalizing may be carried out as shown in myprevious Patent No. 1,536,749. The flanges 16 and 17, when of lowresistance, produce a very effective short circuit turn. If, however,high resistance material is used, the effective screening of thesemembers may be increased by plating or coating with copper or othercoating of high conductivity. Any method which will produce the effectof a short circuit turn outside of the path of the useful flux willaccomplish the desired result whether provided externally orinternallyof the bushing housing.

The advantages of making simultaneous changes in the number of turns inboth primary and secondary windings are very considerable and easilycarried out in low voltage current transformers where a high degree ofaccuracy is required over awide range of current in the primary circuit.The invention is not restricted to any particular volta e.

In the form of the invention illustrated in the drawings, the switchingmechanism is shown as operated from the high tension side of thetransformer, but it is evident that this mechanism may be operated fromthe low as well as the high tension side.

I claim: I 1. A current transformer in which the total current in theprimary circuit is substantial- .ly independent of the secondarycircuit, said transformer having a plurality of primary marily upon theapparatus electrically connected in that circuit rather than upon theload on the secondary circuit of said transformer having a number ofprimary turns, and means controlled by the amount of current flowing inthe primary circuit of the transformer for changing the number of turnsin said'circuit to maintain the ampere turns in said. primary withinpredetermined limits;

3. A'current transformer in which the total current in the primarycircuit depends primarily upon theapparatus electrically connected inthat circuit rather than upon the load on the secondary circuit of saidtransformer having a plurality of primary coils, switching mechanism forconnecting varying numbers of coils in the circuit of said transformer,and a relay controlled by the current flowing through said transformerfor operatingsaid switchingmechanism.

4. A conductor, a current transformer in which the total current in theprimary circuit depends primarily upon the apparatus electricallyconnected in that circuit rather than :upon the load in the secondarycircuit of said transformer having a plurality of primary turns arrangedto beconnected in series with said conductor, switching mechanism forintroducing different numbers of turns in circuit with said conductor,and arelay controlled by the current flowing in said conductor foroperating said switching mechanism.

5; A conductor, a current transformer in which the total current in theprimary circuit depends primarily upon the apparatus electricallyconnected in that circuit rather'than upon the load on the secondarycircuit of said transformer having a number of turns arranged to beconnected in "seri es with said 'con ductor, and-switching mechanism forchanging'the number of turns in circuit with said conductor, saidswitching mechanism including a're'lay responsive to the current in saidconductor for controlling a portion of the turns of'said transformer,and a hand switch for controlling the other turns of said transformer.

6. A current transformer having a plurality of primary and secondaryturns, means for changing the number of turns in the primary circuit ofsaid transformer, and automatically operated means for maintaining asubstantially constant current ratio notwithstanding actuated by thecurrent in said transformerfor changing the number of turns in theprimary, and means also operated by said relay for changing the numberof turns in the secondary to maintain a constant ratio between theprimary and secondary of said transformer.

9. In combination a conductor, a current transformer including aplurality of primary turns in series with said conductor, switchingmechanism for changing the number of turns in the primary circuit ofsaid transformer, and supplemental switching mechanism interconnectedwith the switching mechanism for the primary turns of said transformerfor maintaining a substantially constant current: ratio notwithstandingchanges in the number of primary turns.

10. In combination a conductor, a current transformer having a pluralityof primary turns arranged to be connected in series with said conductor,switching mechanism for changing the number ofturns in the primarycircuit of said transformer, a relay actuated by the current flowingthrough said conductor for operating said switchmg mechanism, aplurality of turns forming the secondary of said transformer, switchingmechanism for changing the number of turns in the secondary circuit ofsaid transformer, and means interconnecting the switching mechanism forsaid primary and secondary turns to produce simultaneous operation ofsaid switching mechanism and maintain a'constant ratio between theprimary and secondary turns of said transformer.

11. A bushing insulator, having a conductor extending therethrough, aplurality of coils disposed in said bushing insulator and forming theprimary circuit of a current transformer, a switch housing mounted onsaid bushing insulator, and switching mechanism disposed insaidhousingandconnected with the coils in said insulator for changingthe number of turns in the primary circuit of said transformer.

12. In combination a bushing insulator having a conductor extendingtherethrough, a current transformer having primary turns disposed insaid insulator, a switch housing mounted on said insulator, switchingmechanism disposed in said housing and connected with the primary turnsof said transformer for changing the number of turns in circuit withsaid conductor, a plurality of secondary turns for said transformerdisposed in said insulator, switching mechanism disposed outside of saidinsulator and connected with the secondary turns of said transformer forchanging the turns in the secondary circuit of said transformer, andmeans interconnecting the primary and secondary switching mechanism forsimultaneous operation.

13. In combination, a conductor, current transformer windings excited byhe current in said conductor, the current in said conductor beingcontrolled primarily by apparatus in circuit therewith rather than bythe load on the secondary of said transformer, a mercury switch forchanging the number of turns of said transformer winding, and a relayresponsive to the current in said conductor for operating said switch.

14. In combination, a conductor, current transformer windings excited bythe current in said conductor, separate switches for changing the numberof turns respectively in the primary and secondary circuits, of saidtransformer, means interconnecting said switches for simultaneousoperation, and a relay controlled by the current in said conductor foractuating said switches.

15. A current transformer having primary and secondary turns and meanscontrolled by the current in the primary for restricting the range ofvariation of ampere turns in said primary and for maintaining a constanttransformation ratio for changes in current in the primary.

16. The combination with a bushing insulator, of a conductor extendingthrough said insulator, a current transformer having primary turns inseries with said conductor, secondary turns for said transformer, aninsulatin transformer in circuit with said. secondary turns, a groundconnection through the primary of said insulating transformer to thesecondary of said current transformer, and electric translating meansenergized by changing current flowing through said grounded connection.

17. A transformer having primary and secondary windings, a core ofmagnetic material for directing the magnetic lines of force of saidtransformer, said core having an air gap therein, and a shield of highlyconductive, n on-magnetic material disposed outside of said windings andof said core for restricting lines of force in the air space outside ofsaid magnetic core.

18. In combination a bushing insulator, a wall of conducting materialhaving an opening through which said insulator extends, a

current transformer disposed within said insulator, and a shield forrestricting the magnetlc field of said transformer to minimize theinfluence on said transformer of the magnetic characteristics of saidwall, said shield comprising a closed ring of conducting materialdisposed outside of the outer winding of said transformer and betweensaid winding and the periphery of the opening in said wall.

19. In combination a bushing insulator, a magnetic core disposed withinsaid insulator, a primary winding surrounding said core, a secondarywinding outside of said primary winding and insulated therefrom, amember of magnetic material within said insulator and outside of saidsecondary winding and forming a return path for the magnetic flux ofsaid transformer, and a closed ring of highly conductive non-magneticmetal disposed about said last named magnetic memher and within saidinsulator for minimizing the eifect of said transformer on magneticmaterial outside of said insulator.

20. A transformer having primary and secondary circuits, and meanscontrolled by the current in one of said circuits for chang ing thenumber of turns in both circuits, said means being constrained to changethe number of turns in the two circuits simultaneously.

21. A current transformer having primary and secondary circuits, andmeans for automatically changing the number ofturns in each circuit whenthe current in one of said circuits reaches a predetermined value.

In testimony whereof I have signed my name to this specification this16th day of October A. D. 1929.

ARTHUR O. AUSTIN.

